Corrosion of metals can be a significant problem where metal surfaces are exposed to water, air, inorganic metal salts, and the like. One very significant source of corrosion is seawater, which can attack and corrode ferrous and non-ferrous metals. A variety of equipment is subject to corrosion by seawater, including piping, dry-docks, and surfaces of ships and barges, particularly the inner surfaces of ballast tanks.
Many corrosion preventive compositions are known. One of the more common approaches to the prevention or inhibition of corrosion is to coat the metal surface with an organic material, thereby placing a barrier between the metal surface and the environment. Frequently, a substantially permanent coating such as a cured resin is utilized. However, in many situations a cured resin coating is impractical. For example, as previously noted, a common site of corrosion problems is the interior of ship ballast tanks. Generally, maintenance or repair work is performed inside these tanks at least once a year. Typically, any coating must be stripped off the inner surfaces of the tank to facilitate the work therein. Many of the cured resin coatings and other rust preventives commonly used for other purposes are inappropriate for use in a ballast tank because such coatings typically require caustic or toxic removers or strippers for their removal. Use of these toxic substances inside the tanks where work must be performed is highly undesirable. Preferably, protective coatings utilized on the interior surfaces of ballast tanks should be removable with a solvent of low volatility and toxicity, to prevent injury to those who must remove the coating or perform maintenance or repair of the tank.
A corrosion or rust preventive coating for ballast tanks which can be removed by solvents of low toxicity is disclosed in U.S. Pat. No. 3,925,087, issued Dec. 9, 1975 to Lechner et al. However, the composition disclosed in U.S. Pat. No. 3,925,087 will generally provide a coating of no greater than 3 mils, or more typically, 1 to 2 mils. When applied in a thicker layer, this and other similar coatings generally fail to properly cure, and to the extent they do cure, require an inconveniently long period of time to do so. Further, thicker coatings of compositions such as that disclosed in U.S. Pat. No. 3,925,087 tend to flow, and in fact a coating thickness of greater than about 3 or 4 mils generally cannot be maintained.
The thinner coatings, such as those from 1 to 3 mils, tend to provide limited protection from corrosion because a thinner coating of known rust preventive coating materials will tend to wear away faster than a thicker coating of an equally effective substance.
Accordingly, a need exists for a corrosion or rust preventive coating composition which will maintain a coating thickness of greater than 3 mils, which is relatively safe and can be relatively easily and safely removed from a surface when desired, and which exhibits exceptionally effective and long lasting corrosion prevention even where applied in a thickness of less than 3 mils.